1. Field of the Invention
The present invention relates to a braking system for use in automotive vehicles and particularly to a hydraulic braking system comprising a dynamic hydraulic pressure control system which outputs powered hydraulic pressure supplied from a powered hydraulic pressure source in response to movement of a braking pedal.
2. Description of the Prior Art
A conventional braking system for automotive vehicles provides a plurality of hydraulic circuits connecting wheel brake cylinders mounted on wheels and a hydraulic pressure control system such as a master cylinder, which is able to achieve a braking operation even if the one of the hydraulic circuits is broken. In general, a tandem master cylinder is used in a conventional dual circuit system.
Also, to reduce the force required to operate the brake pedal in the braking operation, the hydraulic braking system can be provided with a power multiplying device referred to as a servo or booster. Either compressed air, under pressure from an intake pipe, or hydraulic pressure, from a hydraulic booster, is often used as the power multiplying source. A hydraulic booster drives a hydraulic braking pressure control system such as a master cylinder with multiplied force in response to actuation of a braking pedal, with the powered hydraulic pressure output from the hydraulic booster used as the power multiplying source. For instance, Japanese Patent Laid-Open Publication No. 59-209948 has disclosed a system which includes a hydraulic pressure booster connected to a tandem master cylinder which operates as an ordinary tandem master cylinder when the hydraulic pressure booster is not operated.
Along with employment of a hydraulic pressure booster, it has also been proposed to operate the hydraulic pressure booster as a dynamic hydraulic pressure control system in the hydraulic braking pressure control system. Namely, the hydraulic braking pressure (hereinafter referred to as boosted hydraulic pressure) is controlled in response to the brake pedal with powered hydraulic pressure being applied directly to one hydraulic circuit. For example, as disclosed in Japanese Patent Laid-Open Publication No. 59-227552, the boosted hydraulic pressure from the hydraulic pressure booster is applied to the rear wheels of a front and rear wheel split braking system and thereby the stroke of a brake pedal needed for braking can be shortened.
However, such prior art is accompanied by a problem that if the powered hydraulic pressure is decreased due to the cease of operation of the powered hydraulic pressure source, the braking force to the rear wheels disappears.
Also, in view of the necessity of applying a larger braking force to the front wheels than to the rear wheels for stability of the vehicle, it is difficult to connect the front wheels to the dynamic hydraulic pressure control system.